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Gallium nitride (GaN) high electron-mobility transistors (HEMTs) offer considerable high-power operation but suffer in reliability due to potentially damaging self-heating. In this study, self-heating in AlGaN/GaN HEMTs on high conductivity substrates is assessed using a high-resolution thermoreflectance (TR) imaging technique, to compare the thermal response between GaN-on-Si, GaN-on-Diamond, and GaN-on-4H-SiC. The TR method accuracy at high-power density is verified using a nonlinear coefficient of TR (CTR) as a function of temperature. The acquired steady-state thermal maps give a thermal resistance of 11.5~mm ⋅ K/W for GaN-on-Si (based on peak channel temperature), compared to 2.7 and 3.3~mm ⋅ K/W for GaN-on-SiC and GaN-on-diamond substrates, respectively. The tested GaN-on-diamond HEMT exhibits similar heating rates to those seen on a SiC substrate, with a slightly higher peak temperature, which indicates a higher thermal boundary resistance that could offset the benefits of using a higher conductivity substrate and lead to faster thermally enhanced degradation. The analysis reveals the importance using high-resolution imaging to detect hotspots and areas of peak temperature that largely affect failure initiation and device reliability and which may not be otherwise observable. [ABSTRACT FROM AUTHOR]

Record DC power has been demonstrated in AlGaN/GaN high electron mobility transistors fabricated using a substrate replacement process in which a thick diamond substrate is grown by chemical vapor deposition following removal of the original Si substrate. Crucial to the process is a ~30 nm thick SiN interlayer that has been optimized for thermal resistance. The reductions obtained in self-heating have been quantified by transient thermoreflectance imaging and interpreted using 3D numerical simulation. With a DC power dissipation level of 56 W/mm, the measured average and maximum temperatures in the gate-drain access region were 176 °C and 205 °C, respectively. [ABSTRACT FROM AUTHOR]

In this paper, electrical characterization results of N-polar GaN high-electron-mobility transistors that have been released from a 6H-SiC wafer and manually transferred to a Si wafer using a novel epitaxial lift-off (ELO) technique are presented. This recently developed ELO method uses a thin sacrificial layer of Nb2N, a hexagonal epitaxial conductor with less than 1% lattice mismatch to 4H- and 6H-SiC, to serve as the template for III-N device heterostructure growth. Measured results of transferred devices indicate that electron transport properties and low power density electrical performance are nominally unchanged relative to values measured before release. This technique has several advantages over competing ELO techniques, such as the well-known smart cut method, including bonding-ready released material with atomically-smooth backsides (≤ 0.5 nm rms), easy substrate reclaim with indefinite recycling potential, and a transfer process that can be performed after full front-side device processing and yield screening has been completed. [ABSTRACT FROM AUTHOR]

Carrier injection by single-photon absorption using ultraviolet optical pulses is used to investigate single-event transients in pristine and proton-irradiated AlGaN/GaN HEMTs. High-precision spatial mapping of defects and traps in AlGaN/GaN HEMT devices identify regions of enhanced SET signals, or “hot spots” that are ascribed to the presence of lattice defects that modify the electric field in the structure. The resulting transient shape and amplitude depend strongly on the specific location of the injected carriers, and vary with changes in gate and drain bias. Proton irradiation significantly alters transient signal shapes, increases the integrated collected charge, and increases density of defects and traps. [ABSTRACT FROM PUBLISHER]

Radiation tolerance of AlGaN/GaN high-electron mobility transistors (HEMTs) is studied with 2-MeV protons, up to a fluence of 6 \times 10^14 H+/cm2 (about 200 times of typical Si MOSFET rating). The increase in dynamic ON-resistance ( $R_{{\textit{ONDYN}}})$ after radiation is observed to be much more severe than that of static ON-resistance. Radiation-induced donorlike traps located near the two-dimensional electron gas trap electrons, which is responsible for the phenomenon. Compared with the devices passivated by conventional plasma-enhanced chemical vapor deposition (PECVD) SiN, GaN HEMTs with 10 nm of in situ SiN before the PECVD SiN step demonstrate much less increase in $R_{{\textit{ONDYN}}}$ from 2300% to only 300%. The in situ SiN is believed to reduce the process damage by PECVD, improving radiation tolerance. [ABSTRACT FROM PUBLISHER]

A finite-element model coupling 2-D electron gas (2-DEG) density, piezoelectric polarization charge Q{\bf P}, and intrinsic stress induced by a nanocrystalline diamond capping layer, was developed for AlGaN/GaN high electron mobility transistors. Assuming the surface potential is unchanged by an additional stress from diamond capping, tensile stress from the diamond cap leads to an additional tensile stress in the heterostructure and, thus an increase in the 2-DEG under the gate. As a result, additional compressive stress near the gate edges would develop and lead to decreased 2-DEG in the regions between the source and drain contacts (SDCs). Increased saturation drain current will be due to the reduced total resistance between SDC. Integration of the 2-DEG density from SDC revealed a redistribution of sheet density with total sheet charge concentration remaining unchanged. The modeling results were compared with the experimental data from Raman spectroscopy and I-V characterization, and good agreements were obtained. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high-electron mobility transistor (HEMT) structures were grown by metal–organic chemical vapor deposition on SiC, hydride vapor phase epitaxy (HVPE) GaN, and ammonothermal GaN substrates to achieve HEMTs with over five orders of magnitude variation in extended defect density. This enables a direct comparison of the effect of extended defects on device performance to achieve the best possible reliability. As-grown material was characterized by atomic force microscopy, electron channeling contrast imaging, and Raman spectroscopy. Devices were characterized by Hall, dc $I$ – $V$ , and pulsed $I$ – $V$ behavior. Reduced threading dislocation density provides an increased 2-D electron gas mobility, but inhibits ohmic contact formation resulting in high contact resistance. Transistor characteristics were nominally identical, with higher OFF-state leakage in the HEMTs on ammonothermal GaN. The pulsed $I$ – $V$ response indicated significantly reduced current collapse in the HEMT on HVPE GaN due to reduced buffer trapping. [ABSTRACT FROM PUBLISHER]

AlGaN/GaN high-electron mobility transistors (HEMTs) were exposed to 2-MeV protons irradiation, at room temperature, up to a fluence of 6 \times 10^\mathrm \mathbf 14 H+/ \mathrmcm^\mathrm \mathbf 2 . Aside from degradation resulting from radiation-induced charge trapping, transmission electron microscopy and electrical measurements reveal a radiation-induced defect located at the edges of the Ni/Au Schottky gate in the proton-irradiated devices. At the edges of the Ni/Au gate, the Ni of the Ni/Au gate diffused up into the Au layer and migrated into the AlGaN barrier, leaving voids in the Ni layer at the gate edges after irradiation. These radiation-induced voids are caused by diffusion of Ni through vacancy exchange, known as the Kirkendall effect, resulting in reduced gate area and degrading the HEMT performance. [ABSTRACT FROM AUTHOR]

In this split-wafer study, we have compared the dc, pulsed, small and large signal RF electrical performance of nanocrystalline diamond (NCD) coated AlGaN/GaN high electron mobility transistors (HEMTs) to reference devices with silicon nitride passivation only. The NCD-coated HEMTs were observed to outperform reference devices in transconductance, large-signal gain, output power density, and power-added efficiency at 4 GHz. The measured improvements were suspected to be related to reduced dispersion and lower source access resistance afforded by the NCD film. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors grown on Si, SiC, and sapphire substrates were exposed to 2-MeV proton irradiation in incremental fluences up to \(6 \times 10^{14}\) cm \(^{-2}\) . The devices were characterized initially and after each irradiation by Hall and dc \(I\) – \(V\) measurements to probe the mechanisms associated with radiation-induced degradation and failure. It was determined that defects created at the AlGaN/GaN interface introduce scattering centers near the two-dimensional electron gas (2DEG), which result in degraded mobility. Additionally, charged traps in the structure serve to screen the 2DEG resulting in reduced sheet carrier density. These two effects are responsible for degraded \(I\) – \(V\) behavior, including reduced saturation current and transconductance, increased ON-resistance, and positive threshold voltage shift. Interestingly, the sample with the most pre-existing defects was the most tolerant of radiation-induced damage. [ABSTRACT FROM PUBLISHER]

Boron-doped p^+ nanocrystalline diamond (NCD) films are implemented as heat spreading gate contacts to AlGaN/GaN high-electron-mobility transistors. This device demonstrates a reduced ON-resistance, reduced gate leakage, and significantly increased ON-state current density compared with the reference Ni/Au-gated devices from the same wafer. The NCD gate electrode is thermally stable, chemically stable, optically transparent, and places a heat spreading film in direct contact with the gate edge, which is the hottest part of the device. [ABSTRACT FROM AUTHOR]

Enhancements in AlGaN/GaN high-electron-mobility transistor (HEMT) performance have been realized through ultrathin (4 nm) AlN passivation layers, formed by atomic layer epitaxy (ALE). A combination of ex situ and in situ surface cleans prepare the surface for deposition of ALE AlN. HEMTs passivated by high crystallinity AlN, grown at 500^\circC, show improvements in 2-D electron gas sheet carrier density, gate leakage current, off-state drain leakage current, subthreshold slope, and breakdown voltage. In addition, degradation of dynamic on resistance during pulsed off-state voltage switching stress is suppressed by \sim50\% compared with HEMTs passivated by conventional plasma enhanced chemical vapor deposition SiNx. [ABSTRACT FROM PUBLISHER]

Nanocrystalline diamond (NCD) thin films are deposited as a heat-spreading capping layer on AlGaN/GaN HEMT devices. Compared to a control sample, the NCD-capped HEMTs exhibited approximately 20% lower device temperature from 0.5 to 9 W/mm dc power device operation. Temperature measurements were performed by Raman thermography and verified by solving the 2-D heat equation within the device structure. NCD-capped HEMTs exhibited 1) improved carrier density NS, sheet resistance RSH; 2) stable Hall mobility \muH and threshold voltage VT; and 3) degraded on-state resistance RON, contact resistance RC, transconductance Gm, and breakdown voltage VBR. [ABSTRACT FROM PUBLISHER]